A manufacturing method and a control wheel of the type mentioned above are known. During the manufacture of the control wheel, the blades, together with their profile, root and shroud parts, are milled from bar material. The individual blades are welded together at the root and shroud providing a complete ring. This in turn, is welded onto the rotor with a large bell seam by using a submerged arc process. After stress-relief heat treatment, the entire wheel part is turned and the root of the bell seam, in which the initiation of cracks tends to occur, is removed.
The shrouds are welded by this method, into groups each made up of three or more individual blades. These groups are matched before being connected to avoid vibration excitation due to the steam flow emerging from the nozzles. The groups are not connected together at the shrouds.
In order to obtain a closed ring, the individual blades or the groups of blades must be connected together in their root zones. To this end, the radial extent of the root platforms is dimensioned substantially larger than the shrouds. The weld seams do not extend to the wall of the root platform forming the flow boundary. In consequence, the root zones of the weld seams at the blade roots are removed by drillings at the periphery of the disk, which gives the welded control wheel its characteristic appearance.
Currently the materials used are difficult to weld, an example being X.sub.22 CrMoV.sub.121. Nevertheless, welded connections have to be produced having the same strength and ductility as the basic material. For this reason, the weld material must be subjected to heat treatment. However, the weld material used, which must have a high C content so that it does not become two-phase, has the property that it does not slowly transform at preheating temperatures of about 300.degree. C., as low-alloy steels do; it remains austenitic and can only be transformed by cooling. For this reason, the control wheel is normally cooled after it has been welded together so that the desired transformation takes place. The brittle martensite which results is subsequently made tougher and more ductile by annealing.
A disadvantage in this solution is the fact that the welds are subject to the danger of cracking during the cooling phase mentioned. Although the transformation is improved with additional cooling to lower temperatures, and improved ductility is achieved during subsequent annealing, the danger of cracking increases quite substantially. For this reason, it has not, until now, been possible to weld the shrouds together to form a closed ring.
Another disadvantage of known control wheels is that the roots of the weld seam in the root platforms cannot be inspected, therefore, at least the root seam, which is subject to cracking, must be bored out for safety reasons.